The present invention relates to an arrangement for providing upstream transmission of packet voice data through an HFC network and, more particularly, to the inclusion of a synchronization unit in a subscriber""s broadband terminal interface unit to control the timing of the creation and transmission of upstream packets with respect to a received upstream grant.
As originally designed and implemented, cable television systems used only coaxial cable between a head-end controller (or distribution station) and the customer. Newer cable systems tend to be hybrids of optical fibers and coaxial cable; optical fiber being used for the long-haul portion of the network and the coaxial cable being used to bring the communication from xe2x80x9ccurbsidexe2x80x9d into a home or office (in general, distances of less than a few thousand feet or so are spanned by a coaxial cable in such a network). FIG. 1 shows a hybrid fiber-coax (HFC) system 10 comprising a head-end controller 12 connected by one or more fiber optic links 14 to a fiber-to-coax conversion unit 16 which, in turn, feeds a coaxial cable distribution network 18 running into a number of individual user or customer sites (e.g., homes 20 in FIG. 1).
Cable systems originally provided unidirectional transfer of programming from head-end controller 12 to a home 20. With the growth of computer networks and a desire to provide real-time interactive services to the customer, the need has arisen for an efficient means to provide two-way (bi-directional) communication over an existing HFC network. A number of service providers currently furnish two-way services over cable. In these services, there is generally a modestly high bandwidth link from the head-end controller to the user site. However, the link in the other direction (i.e., an xe2x80x9cupstreamxe2x80x9d link from the user""s site to the network) is generally through conventional dial-up facilities such as a telephone modem or an integrated services digital network (ISDN) connection, which exhibits a much lower bandwidth that the downstream cable link. Such systems allow the customer to quickly download material from the network into their local machine (i.e., personal computer or any other type of terminal equipment), but does not readily permit the user to originate transmissions requiring high bandwidth (such as voice communications).
One way of enabling customers to originate voice communication is to permit them to use the cable system, which has a comparatively high bandwidth. Within existing HFC networks, providing customers with the ability to transmit data upstream requires service providers (i.e., those organizations that operate head-end equipment) to reserve sections of the cable spectrum (bandwidth) or time slots for the upstream communication. When used with voice communications however, long delays in the upstream packet transmission (as well as packet jitter) have a negative effect on the audible voice quality. In a traditional voice coding approach, the encoding of the PCM samples occurs at the end of the packetization period. The problem with the traditional approach is that it is difficult to synchronize the resultant encoded packets to the rest of the upstream communication system. In the prior art, the sample accumulation, encoding and packetization ran off of its own clock. Therefore, the xe2x80x9cbestxe2x80x9d possible synchronization that could be guaranteed had a resolution of the packetization time.
Thus, a need remains in the prior art for an improved system of transmitting upstream packet voice traffic that minimizes the inherent packet characteristics (e.g., latency, jitter) that are particularly noticeable during voice communications.
The need remaining in the prior art is addressed by the present invention, which relates to an arrangement for improving the upstream transmission of packet voice data through an HFC network and, more particularly, to the inclusion of a synchronization unit in a subscriber""s broadband terminal interface unit to control the timing of the creation and transmission of upstream voice packets with respect to a received upstream grant.
In accordance with the present invention, a programmable logic device (PLD) and phase-locked loop (PLL) are used to provide a number of different timing and interrupt signals for the remaining components in the BTI. The PLL uses timing information from the cable modem termination system (CMTS) to generate a base clock signal, which is then supplied to the PLD. The PLD, in turn, uses this clock to create a sampling clock signal that is supplied as an input to the codec used to create the PCM data packets of a to-be-transmitted voice signal. The PLD provides both an xe2x80x9cinterruptxe2x80x9d signal and a xe2x80x9cframe syncxe2x80x9d signal to the DSP, while simultaneously and in synchronization with the DSP interrupt signal, supplying a xe2x80x9chost packet interruptxe2x80x9d signal to the host microprocessor. Therefore, upon the host microprocessor receiving an upstream grant (UG) signal, it will send an xe2x80x9copen channelxe2x80x9d command to the DSP, along with information regarding which frame within the packet interval to use to begin the transmission. Since the host microprocessor uses the UG signal to initiate the process and utilizes the same timing and interrupts as the DSP, the transmission of packets from the DSP to the host DSP interface will be controlled and synchronized with the UG signal.
These and other features and objects of the present invention will be more fully understood from the following detailed description of the preferred embodiments which should be read in light of the accompanying drawings.